In a phase change optical information recording medium used as a rewritable medium, information can be recorded, erased and rewritten by utilizing a recording layer in which phase change is caused reversibly between a crystalline phase and an amorphous phase. When such a recording layer is irradiated with a laser beam at a high power and then cooled, the irradiated portion becomes an amorphous phase. Furthermore, when the amorphous portion of the recording layer is irradiated with a laser beam at a low power and then gradually cooled, the irradiated portion becomes a crystalline phase. Therefore, in the phase change optical information recording medium, the recording layer can be changed arbitrarily into the amorphous phase or the crystalline phase by irradiating the recording layer with a laser beam whose power is modulated between a high power level and a low power level. In such an optical information recording medium, information is recorded by utilizing a difference between the reflectance in the amorphous phase and the reflectance in the crystalline phase.
In recent years, in order to improve the recording density of the optical information recording medium, various techniques have been researched. For example, the following techniques have been researched: a technique by which a smaller recording mark is recorded by using a blue-violet laser beam having a relatively short wavelength; and a technique by which a smaller recording mark is recorded by reducing the thickness of a substrate provided on the laser beam incident side and using a lens having a large numerical aperture (NA). Moreover, there is a technique by which two information layers, each of which include a recording layer, are provided, and information is recorded and reproduced on each of the two information layers by using a laser beam incident from one side (e.g., JP2000-36130A).
FIG. 4 shows an example of a structure of a conventional optical information recording medium in which two information layers are provided. In the conventional optical information recording medium 101, a first information layer 103, an optical separating layer 104 and a second information layer 105 are provided in this order from the laser beam, incident side between a first substrate 102 and a second substrate 106. In the first information layer 103, a protective layer 1031, a recording layer 1032, a protective layer 1033, a reflective layer 1034 and a transmittance adjusting layer 1035 are provided in this order from the laser beam incident side.
Thus, in the optical information recording medium 101 in which information is recorded and reproduced in the two information layers by irradiation of a laser beam from one side, recording and reproduction of the second information layer 105 provided on the side opposite to the laser beam incident side is performed by using a laser beam that has been transmitted through the first information layer 103 provided on the laser beam incident side.
In order to perform recording and reproduction in such an optical information recording medium 101, it is preferable that the first information layer 103 has a transmittance as high as possible. Therefore, in the first information layer 103, a transmittance adjusting layer 1035 made of a dielectric material having a high refractive index is provided in contact with the reflective layer 1034 in order to increase the transmittance. As a dielectric material having a high refractive index, for example, TiO2, Nb2O5 and materials containing these can be used.
The optical information recording medium 101 provided with the transmittance adjusting layer 1035 as described above generally is manufactured in the following process order to facilitate film-formation.
(a) a process of forming the second information layer 105 on the second substrate 106.
(b) a process of forming the optical separating layer 104 on the second information layer 105.
(c) a process of forming the first information layer 103 on the optical separating layer 104.
(d) a process of attaching the first substrate 102 onto the first information layer 103.
In order words, in the conventional optical information recording medium 101, when forming the first information layer 103 on the optical separating layer 104 in the process (c), first, the transmittance adjusting layer 1035 is formed on the optical separating layer 104, using a dielectric material having a high refractive index.
However, when the inventors produced the transmittance adjusting layer 1035 with a single-wafer film-forming apparatus for single wafer processing having a plurality of film-formation chambers, the following was made evident. When forming the transmittance adjusting layer 1035 with a dielectric material having a high refractive index in a first film-formation chamber, since the dielectric material having a high refractive index is very sensitive to an atmosphere for film formation, the film-forming rate tends to be varied by the influence of, for example, water contained in a base material (the state in which the second information layer 105 and the optical separating layer 104 are formed on the second substrate 106). When a load lock chamber, which is the first chamber when a base material is introduced to a film-formation chamber, is kept in a vacuum for a long time in order to solve this problem, the variation in the film-forming rate can be suppressed. However, in view of productivity, this is not preferable because when a vacuum is kept for a long time, the film-forming cycle takes a long time.